The present invention is directed to an arrangement for stabilizing the idling speed of an internal combustion engine by early adjustment of an ignition angle. The arrangement is of the type which has a rotating encoder device, such as a distributor, that supplies encoder signals proportional to the speed of the internal combustion engine, a motor control device that contains a microcomputer to which the encoder signals are supplied and that provides a premature angle therefrom as the ignition angle for a defined idling speed of the internal combustion engine.
Such an early adjustment of the ignition angle is disclosed, for example, by German Published Application 30 08 232. As disclosed, a microcomputer calculates the ignition angle as a function of parameters such as the RPM of the internal combustion engine.
Calculating speed is a problem for microcomputers in motor control devices which calculate the blocking period for every cylinder of an internal combustion engine. The time available to the microcomputer between two ignitions in order to calculate the optimum ignition angle becomes less for higher speed of the internal combustion engine, for more cylinders and for more functions of the engine and the more functions the motor control must satisfy. This problem is present whether or not the microcomputer calculates the ignition angle before every ignition or derives it from performance characteristics.
In order to chronologically reduce the demands on the microcomputer, the calculation of the ignition angle can be done, for example, for only every 4th ignition instead of for every individual ignition. The ignitions between two calculations then all have the same ignition angle that may not be optimum under certain conditions. This procedure is therefore less suitable in operating ranges of the internal combustion engine that require an extremely fast adaptation of the ignition angle. During a fast reduction of the RPM when the internal combustion engine is idling, for example, this reduction caused by a sudden increase in load, an early adjustment of the ignition angle is necessary at the next following ignition in order to prevent the engine from stalling.
In this case, there is only the possibility of either using a correspondingly fast and, thus, expensive microcomputer or of accepting a less optimum idling stabilization.